Conventionally, a heat roller fixing method has been generally used in a fixing apparatus incorporated into an image forming apparatus using an electrophotographic process. A copying machine and a printer are examples of such image forming apparatus. In the heat roller fixing method, a heated fixing roller and a pressure roller are press-contacted to each other. A paper sheet (recording material) holding an unfixed toner image thereon passes between the fixing roller and the pressure roller so that the toner image is fused and fixed on the paper sheet.
Recently, the image forming apparatus has been desired to carry out processes at higher speed. Accordingly, the number of sheets processed per unit of time increases and speed at which the sheets are transported becomes higher. For example, an apparatus which processed 60 sheets per minute (horizontal transportation of A4 sheets) was regarded as a high speed apparatus conventionally, but an apparatus which processes 80 or more sheets per minute is regarded as a high speed apparatus recently. Nowadays, an apparatus which can process 100 or more sheets per minute has been developed.
A fixing apparatus incorporated into such a high speed apparatus adopts a mode (external heat fixing mode) in which external heating means is provided on an external periphery of the fixing roller and the fixing roller is heated from the outside (from the surface side) in order to quickly heat the surface of the fixing roller and reduce total power consumption of the whole apparatus.
As the external heating means, there are proposed (i) a belt type in which an endless belt is suspended between a plurality of heating rollers each of which has a heat source therein so that the heated belt member is brought into contact with the fixing roller and (ii) a roller type in which a heating roller (hereinafter, referred to as “external heating roller”) is brought into contact with a surface of the fixing roller.
Generally, the external heating roller is arranged so that a heater serving as a heat source is disposed in an extremely thin cylinder having an excellent heat conductivity and made of aluminum, iron material, or the like. The thinness of the external heating roller allows its surface to be quickly heated with a heat amount supplied from the heater, so that it is possible to quickly heat the surface of the fixing roller by bringing the external heating roller into contact with the fixing roller (quick heating property).
[PATENT DOCUMENT 1]
Japanese Unexamined Patent Publication No. 190294/1996 (Tokukaihei 8-190294)(Publication date: Jul. 23, 1996)
However, the conventional fixing apparatus having the external heating roller raises such a problem that: after finishing the step of heating the fixing roller, e.g., after finishing the fixing roller warm up step and the fixing step, the fixing roller is locally heated in a portion which is in contact with the external heating roller, so that the surface of the fixing roller is deteriorated or broken.
A temperature of the fixing roller is controlled by electrifying and stopping electrifying the heaters of the fixing roller and the external heating roller, e.g., by electrifying the heaters until a surface temperature of the fixing roller rises so as to be equal to a preset fixing temperature and by stopping electrifying the heaters when the surface temperature rises so as to be equal to the preset fixing temperature.
While, the fixing roller is driven in the following manner. In the temperature raising step of heating the fixing roller, e.g., in the warm up step, the fixing roller is rotated until the surface temperature of the fixing roller becomes equal to the preset fixing temperature, and the rotation of the fixing roller is stopped when the surface temperature becomes equal to the preset fixing temperature. Further, in the step of keeping the temperature, e.g., in the standby step which is carried out after the warm up step and before receiving an instruction to carry out the printing process, the fixing roller stops. The pressure roller and the external heating roller are arranged so as to be rotated by being brought into contact with the surface of the fixing roller, and rotations of the pressure roller and the external heating roller are stopped by stopping the rotation of the fixing roller.
FIG. 11 shows a conventional fixing apparatus and illustrates turning ON/OFF of each heater, ON/OFF of a driving source of a fixing roller, and variation of surface temperature of each roller, in shifting to a standby mode through a warm up step on start up.
The electrifications of the heaters of the fixing roller, the pressure roller, and the external heating roller are synchronously controlled during warm up, and the heaters are turned OFF when the surface temperature of the fixing roller becomes equal to the preset fixing temperature and the warm up is finished, and then they become in a standby mode. During the standby mode, the heater of the external heating roller is kept OFF, and only the heaters of the fixing roller and the pressure roller are turned ON when the surface temperature of the fixing roller is lower than the preset fixing temperature and are turned OFF when the surface temperature is higher than the preset fixing temperature. Note that, in FIG. 11, a surface temperature of each roller keeps on rising after turning each heater OFF because of time loss of heat conduction in the roller.
Meanwhile, the fixing roller is turned ON after beginning the warm up and when the surface temperature of the fixing roller exceeds a toner softening temperature. Thereafter, the fixing roller is turned OFF when the surface temperature of the fixing roller becomes equal to the preset fixing temperature and the warm up is finished, and the fixing roller is kept OFF in the standby mode.
As illustrated in FIG. 11, in driving the heater and the driving source in the conventional manner, overshoot which is sudden rise of the surface temperature of the external heating roller occurs right after finishing the warm up. That is, as described above, the external heating roller is so thin as to quickly heat the fixing roller, and a temperature gradient between the heat source of the external heating roller and the fixing roller is set so as to be great. Thus, although the heater of the external heating roller is turned OFF, heat conduction from the external heating roller to the fixing roller is instantly cut off when the rotation of the fixing roller is suddenly stopped. As a result, the surface temperature of the external heating roller suddenly rises. When the temperature exceeds an endurable temperature of a surface layer of the fixing roller, the fixing roller is locally heated in a portion which is in contact with the external heating roller, so that the fixing roller is deteriorated or broken.